When I heard the story about the McDonald's Happy Meal that wouldn't decompose, I was sure there would be a smoking gun. If we are to believe her, Sally Davies, a photographer from New York, bought a Happy Meal – a burger and fries – on 26 April. Then, she says, she left it alone on a plate.

She photographed the meal every day. Weirdly, it didn't seem to rot. It didn't smell bad. There were no maggots. Neither the burger nor the fries developed a coating of slime or mould. In the pictures, the burger doesn't change much, from day one right up to day 180.

Could this be true? Well, it was certainly something a lot of people wanted to be true. Davies's pictures, and videos of Davies being interviewed, have become a viral hit on the internet. Davies says she was inspired by another woman, Karen Hanrahan, who has kept a similar McDonald's burger since 1996. That's 14 years.

Another woman, Julia Havey, who describes herself as an obesity activist, uses an ageing burger as a promotional tool. Having been seriously overweight, Havey gave up eating junk food, and keeps a three-year-old McDonald's burger and some ancient McDonald's fries in a lunchbox. There is, as she points out, no mould or mildew on the burger, and the fries look as good as new. "It's ageing better than me," she has said. Havey also keeps pieces of ordinary potato, which turn black after a few weeks. Talking about junk food, she recently said: "Once it gets in your thighs, there's no breakdown! It doesn't go anywhere! Real food breaks down. This can't be real food."

For their part, McDonald's have issued a statement in response to the Davies burger. "It is not possible to provide a detailed explanation regarding these claims without knowing the conditions in which these food items were kept." The fast food giant also points out that their burgers are made from "100% USDA-inspected beef cooked and prepared with salt, pepper and nothing else", and that their "world-famous French fries" are "made from potatoes and cooked in a canola-oil blend".

Three women; three mummified burgers. So what's going on? Well, the mummified burgers and fries are easy to explain. As the food scientist Barry Swanson, from Washington State University, recently told Salon.com: "Anything high in fat will be low in moisture." These French fries are cooked at a very high temperature. They are very thin, and have been fried to a crisp, so they contain lots of fat. They also contain salt, another natural preservative. And what about the burger? Another food scientist, Sean O'Keefe from Virginia Tech, told Salon something similar. "It's also very thin, which once again means high heat per surface area."

This food, then, is not particularly healthy. The fries are made of cooked starch, which quickly turns to glucose in your blood, and salt, which can raise your blood pressure, and fat, which is high in calories – a triple-whammy of badness. The burger – well, it's made of meat from a lot of different cows. And this meat comes from all over these cows. It's not prime fillet steak. And it's not exactly organic. But really, it's just a thin bit of ground-up cow, with added salt, cooked at a high temperature. It doesn't rot like a homemade burger would. This is, basically, because it's not as nice as a homemade burger. Germs don't like it very much.

Perhaps the most disturbing thing about the phenomenon of the immortal junk food, whether it's true or not, is how much we want it to be true. The fact is, we're fat. We're getting fatter. And we really want to blame somebody. So we blame McDonald's. We want to believe that McDonald's is the reason we're fat. But the truth is much more complicated, and much more inconvenient. We are greedy. Our whole culture is greedy. We are fat because our whole culture is based on making us want more stuff all the time. The problem is not just junk food. It's everything else as well.